1. Field of the Invention
The invention generally relates to the field of signal processing for wireless communications. More specifically the invention is related to interference cancellation in single- and multi-mode communication systems.
2. Discussion of the Related Art
In order to efficiently utilize time and frequency in a communication system, multiple-access schemes are used to specify how multiple users or multiple signals share a specified time and frequency allocation. Spread-spectrum techniques may be used to allow multiple users and/or signals to share the same frequency band and time interval simultaneously. Time division multiple access (TDMA) and frequency division multiple access (FDMA) assign unique time or frequency slots to the user. Code division multiple access (CDMA) assigns a unique code to differentiate each signal and/or user. The codes are typically designed to have minimal cross-correlation to mitigate interference. However, multipath effects introduce cross correlations between codes and cause CDMA systems to be interference-limited.
Multiple-access coding specified by TDMA, FDMA, or CDMA standards provides channelization. In a typical CDMA wireless telephony system, a transmitter may transmit a plurality of signals in the same frequency band by using a combination of scrambling codes and/or spreading (i.e., orthogonalizing) codes. For example, each transmitter may be identified by a unique scrambling code or scrambling-code offset. For the purpose of the exemplary embodiments of the invention, scrambling may denote encoding data with a W-CDMA scrambling code or encoding data with short pseudo-noise (PN) sequences, such as used in CDMA2000 and IS-95 systems.
A single transmitter may transmit a plurality of signals sharing the same scrambling code, but may distinguish between signals with a unique orthogonalizing spreading code. Spreading codes, as used herein, encode the signal and provide channelization of the signal. In W-CDMA, orthogonal variable spreading factor (OVSF) codes are used to spread data for multiple access. CDMA2000 and IS-95 employ Walsh covering codes for multiple-access spreading.
While CDMA signaling has been useful in efficiently utilizing a given time-frequency band, multipath and other channel effects cause these coded signals to interfere with one another. For example, coded signals may interfere due to similarities in codes and consequent correlation. Loss of orthogonality between these signals results in interference, such as co-channel and cross-channel interference. Co-channel interference may include multipath interference from the same transmitter, wherein a transmitted signal propagates along multiple paths that arrive at a receiver at different times. Cross-channel interference may include interference caused by signal paths originating from other transmitters.
Interference degrades communications by causing a receiver to incorrectly detect received transmissions, thus increasing a receiver's error floor. Interference may also have other deleterious effects on communications. For example, interference may diminish capacity of a communication system, decrease the region of coverage, and/or decrease data rates. For these reasons, a reduction in interference can improve reception of selected signals.
Multipath and other forms of interference inherently limit the performance and capacity of other types of transmission protocols. For example, Orthogonal Frequency Division Multiplexing (OFDM) and Time Division Multiplexing (TDM) may be interference-limited both in uplink and downlink communications.
Multi-mode transceivers support more than one transmission protocol. For example, a wireless handset may support CDMA, Global Standard for Mobile Communication (GSM), and an OFDM wireless local area network protocol. Furthermore, a wireless handset may support a variety of implementations of a particular transmission protocol. Since different communication systems may employ different parameters for designing multiple-access channels, the nature of interference between systems can vary greatly. Thus, a multi-mode transceiver may employ a wide variety of interference-mitigation strategies. Alternatively, a multi-mode transceiver may employ a single interference-mitigation technique adapted to each mode.